wp_bootstrap_navwalker Fatal Error

As hinted at by the comment from the_dramatist, it is not enough to simply add the nav walker class file to your file structure. You must also load the file somewhere, typically in the theme’s functions file or a common include.

Your nav walker class, path and filename will vary so update accordingly:

require get_template_directory() . '/inc/main-header-navigation-menu-walker.php';